Recently, as batteries which are increasingly expected to be smaller, lighter and have higher capacity, secondary batteries based on non-aqueous electrolysis solution systems such as lithium ion batteries have been proposed and provided in practical use.
In lithium ion batteries, lithium ions can be reversibly inserted and desorbed. That is, lithium ion batteries are formed of a positive electrode, a negative electrode, and a non-aqueous electrolyte, with a property that lithium ions can be inserted and desorbed.
As the material of the negative electrode of the lithium ion battery, a carbon-based material or a metal oxide containing Li having the property that lithium ions can be reversibly inserted and desorbed, such as lithium titanate (Li4Ti5O12), are generally used as a negative electrode active material.
As the material of the positive electrode of the lithium ion battery, a metal oxide containing Li having a property that lithium ions can be reversibly inserted and desorbed, such as lithium iron phosphate (LiFePO4) as a positive electrode active material, is used. An electrode material mixture including the positive electrode active material, a binder, an organic solvent and the like is applied onto a surface of a metal film called a collector, thereby forming a positive electrode of the lithium ion battery.
Lithium ion batteries as described above are light and small and have high energy as compared with secondary batteries such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries in the related art. Accordingly, lithium ion batteries are used as the power supply of mobile electronic apparatuses such as mobile phones and laptop computers. Recently, lithium ion batteries have been examined as a high power supply for electric cars, hybrid cars, electric tools, and the like, and batteries used for them are required to have high-speed charging and discharging properties. However, the electrode material including the electrode active material, for example, including a Li-containing metal oxide having the property that lithium ions can be reversibly inserted and desorbed, has a problem that electron conductivity is low.
Thus, an electrode material with improved electron conductivity has been proposed. For example, an electrode material is disclosed where a secondary particle includes a plurality of primary particles of an electrode active material represented by the formula LixAyBzPO4 (in the formula, A is at least one kind selected from Cr, Mn, Fe, Co, Ni, and Cu, and B is at least one kind selected from Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, Y, and rare-earth elements, where 0≦x<2, 0<y<1.5, and 0≦z<1.5), and the surface of the electrode active material is covered with a carbonaceous coating film so that carbon as an electron conductive material is interposed between the primary particles. As a method for producing the electrode material, a method is disclosed where slurry including an electrode active material or a precursor of the electrode active material, and an organic compound is sprayed, it is dried to generate a granular material, and a heat treatment is performed on the granular material under a non-oxidizing atmosphere at 500° C. or higher and 1000° C. or lower (e.g., see Patent Documents 1 to 3).
However, the electrode material, that is, the electrode material where the secondary particle is an aggregate of the plurality of primary particles of the electrode active material represented by the formula LixAyBzPO4, and carbon is interposed between the primary particles to cover the surface of the electrode active material with the carbonaceous coating film, is required to further improve the conductivity in order to achieve the high-speed charging and discharging properties and the like as described above. That is, the conductivity of the carbonaceous coating film is required to be further improved in order to achieve further sufficient electron conductivity.
In order to improve the conductivity of the carbonaceous coating film, for example, it is conceivable that an organic compound is carbonated at a high heat treatment temperature to raise the graphitization degree of the carbonaceous coating film. However, in this method, the specific surface area of the carbonaceous coating film is also increased by the increase of the heat treatment temperature. Generally, a paste-type electrode material mixture including an electrode material, binder resin (binder) for bringing particles of the electrode material into contact with each other, a solvent, and the like are used at the time of forming the positive electrode of a battery. However, when the specific surface area of the electrode material is increased, the amount of necessary binder resin and solvent is increased. As a result, there is a problem where the application performance of the electrode material mixture deteriorates.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-014340
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-014341
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-015111